In 2012, the Council of Conservative Citizens started seeing a spike in traffic to its website. George Zimmerman was on trial for killing Trayvon Martin, an unarmed black teenager, after a confrontation in their Florida subdivision, and white supremacy organizations like the CCC had become vocal online defenders of Zimmerman’s innocence. Postings on the high-profile case attracted new visitors, and some of the curious became converts.

“When George Zimmerman was acquitted, white nationalists in the U.S. considered that to be a win for them,” says Stephen Piggott, who tracks white supremacist groups for the Southern Poverty Law Center. “The Council of Conservative Citizens’ website blew up. More people were going to the site, and Dylann Roof was one of those people.”

Since the Zimmerman trial, the CCC has been squarely focused on what it calls an epidemic of black-on-white crime. And when Roof Googled that phrase, he found the group’s…